The present invention relates to a melt-extrudable thermoplastic composition which contains an additive system. The composition, when melt-extruded to form a nonwoven web, results in a web having improved tensile strength characteristics or in a wettable web which does not become significantly less wettable over time.
Thermoplastic compositions are described in U.S. Pat. No. 4,923,914 to Ronald S. Nohr and J. Gavin MacDonald, which patent is incorporated herein by reference. The patent describes a surface-segregatable, melt-extrudable thermoplastic composition which comprises at least one thermoplastic polymer and at least one additive having at least two moieties, A and B, in which:
(A) the additive is compatible with the polymer at melt extrusion temperatures but is incompatible at temperatures below melt extrusion temperatures, but each of moiety A and moiety B, if present as separate compounds, would be incompatible with the polymer at melt extrusion temperatures and at temperatures below melt extrusion temperatures;
(B) moiety B has at least one functional group which imparts to the additive at least one desired characteristic;
(C) the molecular weight of the additive is in the range of from about 400 to about 15,000; and
(D) the weight ratio of the polymer to the additive is in the range of from about 1 to about 1,000;
with the proviso that the additive cannot be a compound having the general formula, ##STR1## in which each R independently is a monovalent organic group selected from the group consisting of alkyl groups; R.sub.1 is a monovalent organic group containing at least one ethyleneoxy group, vicinal epoxy group, or amino group; and a and b, which can be the same or different, each have a value of at least 1. In preferred embodiments, the additive is a siloxane-containing compound, and one of the desired characteristics is wettability by water when the polymer is inherently hydrophobic. PA1 (b) R.sub.10 is hydrogen or a monovalent C.sub.1 -C.sub.3 alkyl group; PA1 (c) m represents an integer of from 1 to about 4; PA1 (d) n represents an integer of from 0 to about 3; PA1 (e) the sum of m and n is in the range of from 1 to about 4; PA1 (f) p represents an integer of from 0 to about 5; PA1 (g) x represents an integer of from 1 to about 10; PA1 (h) y represents an integer of from 0 to about 5; PA1 (i) the ratio of x to y is equal to or greater than 2; PA1 (j) said additive has a molecular weight of from about 350 to about 1,400; and PA1 (k) said additive is present in an amount of from about 0.5 to about 5 percent by weight, based on the amount of thermoplastic polyolefin. PA1 (A) melting a mixture which comprises a thermoplastic polyolefin and an additive; PA1 (B) forming fibers by extruding the resulting melt through a die at a shear rate of from about 50 to about 30,000 sec.sup.-1 and a throughput of no more than about 5.4 kg/cm/hour; PA1 (C) drawing said fibers; and PA1 (D) collecting said fibers on a moving foraminous surface as a web of entangled fibers; PA1 (A) said first component is a polysiloxane polyether having the general formula, ##STR3## in which: (1) R.sub.1 -R.sub.9 are independently selected monovalent C.sub.1 -C.sub.3 alkyl groups; PA1 (B) said second component is a hydrophobic fumed silica, in which the weight ratio of said first component to said second component is in the range of from about 20 to about 300.
The compositions described in that patent are particularly useful for the formation of nonwoven webs by such melt-extrusion processes as meltblowing, coforming, and spunbonding. Upon being melt-extruded, such compositions result in a fiber having a differential, increasing concentration of the additive from the center to the surface thereof, such that the concentration of additive toward the surface of the fiber is greater than the average concentration of additive in the more central region of the fiber and imparts to the surface of the fiber at least one desired characteristic which otherwise would not be present. The additive is miscible with the polymer at melt extrusion temperatures, under which conditions the additive and the polymer form a metastable solution. As the temperature of the newly formed fiber drops below melt extrusion temperatures, the additive becomes significantly less compatible with the polymer. Concurrent with this marked change in compatibility, the polymer begins to solidify. Both factors contribute to the rapid migration or segregation of the additive toward the surface which takes place in a controllable manner.
When the additive was a siloxane-containing compound and the desired characteristic was water-wettability, the resulting nonwoven webs, depending upon the additive and/or additive level, could become less wettable over time. This loss of wettability, or aging, was accelerated when the polymer composition contained titanium dioxide. Although the teaching of U.S. Pat. No. 4,923,914 represents a significant improvement over prior methods of imparting water-wettability to shaped articles, e.g., nonwoven webs, made from inherently hydrophobic polymers, the aging problem was a limitation on the usefulness of surface-segregatable compositions, particularly for disposable absorbent products. A subclass of the additives encompassed by U.S. Pat. No. 4,923,914 subsequently was discovered which permitted the preparation of wettable nonwoven webs which remained wettable for at least about two years at ambient temperature. Such subclass is described and claimed in application Ser. No. 07/566,589, entitled SURFACE-SEGREGATABLE COMPOSITIONS AND NONWOVEN WEBS PREPARED THEREFROM and filed Aug. 13, 1990 in the names of Ronald S. Nohr and J. Gavin MacDonald.
Application Ser. No. 07/566,589 provides a surface-segregatable, melt-extrudable thermoplastic composition which comprises at least one thermoplastic polyolefin and at least one additive having the general formula, ##STR2## in which: (a) R.sub.1 -R.sub.9 are independently selected monovalent C.sub.1 -C.sub.3 alkyl groups;
Such application also provides a method for preparing a wettable nonwoven web which remains wettable after its formation for at least two years at ambient temperature, which method comprises:
in which the additive meets all of the requirements just described relative to the composition.
It subsequently was discovered that the use of an additive system comprising certain of the additives of U.S. Pat. No. 4,923,914, including some of the additives of application Ser. No. 07/566,589, and a modified fumed silica having a hydrophobic surface unexpectedly gave either of two results: (1) the amount of additive required in application Ser. No. 07/566,589 can be reduced by up to roughly 50 percent without adversely affecting the wettability of the resulting nonwoven web, or (2) when the additive is a polysiloxane polyether having a molecular weight of at least about 3,000, the resulting nonwoven web is not wettable, i.e., it remains hydrophobic, but exhibits improved tensile strength characteristics as compared with a nonwoven web prepared from a thermoplastic composition lacking the hydrophobic fumed silica.
Silica and other materials have been incorporated into thermoplastic polymers, including polypropylene. For example, the inclusion of an organic peroxide and a nucleating agent in polypropylene is described in Japanese Patent Publication No. 60-217207. The nucleating agent can be an organic salt, an organic compound, or an inorganic material such as silica, alum, titanium dioxide, carbon black, and various clay minerals.
References which describe the inclusion in polypropylene or other thermoplastic polymer of an organic material include U.S. Pat. No. 3,207,735 to Wijga (benzoic acid, substituted benzoic acids, hexahydro analogs thereof, and related compounds), U.S. Pat. No. 3,207,737 to Wales (aluminum salts of benzoic acid and related compounds), U.S. Pat. No. 3,207,739 to Wales (Group I and II metal salts of certain mono- and polycarboxylic acids), U.S. Pat. No. 3,299,029 to Binsbergen et al. (aluminum salts of benzoic acid and related compounds), U.S. Pat. No. 4,611,024 to Wolfe (an acetal of an alditol and a hydrotalcite), and U.S. Pat. No. 4,808,650 to Titus et al. (fluorinated dibenzylidene sorbitol additives); and Japanese Patent Publication No. 51-22740 (benzylidene sorbitol).
Finally, studies relating to the heterogeneous nucleation of polymers have been reported. Examples of such studies include Chatterjee and Price, "Heterogeneous Nucleation of Crystallization of High Polymers from the Melt. I. Substrate-Induced Morphologies", J. Polym. Sci., 13, 2369 (1975); Collington, "The Nucleation of Crystalline Olefins", Polypropylene: The Way Ahead, a conference of the Plastics and Rubber Institute, Madrid, Spain, November 1989; and Garg and Stein, "Crystallization and Morphology of Nucleated Polymers", Antec '88, 1021.